Lithium ion capacitors that combine advantages of a lithium ion battery and advantages of an electric double-layer capacitor are developed as high-capacitance capacitors (with a capacitance of 500 F or more, for example). In recently developed lithium ion capacitors, in general, activated carbon is used as a positive active material, and a carbon material capable of occluding and releasing lithium ions is used as a negative active material. The potential of the negative electrode is kept lower (at approximately −3 V) in the lithium ion capacitor than in the normal electric double-layer capacitor (at approximately −1 V to −1.35 V) since lithium ions are occluded or doped in a negative electrode in advance. Therefore, the cell can be used in a higher voltage range (approximately 2.2 V to 3.8 V). In addition, because the lithium ion capacitor can also serve as a positive electrode charge/discharge mechanism that can adsorb positive ions, in addition to adsorbing negative ions which are utilized by the normal electric double-layer capacitor, the lithium ion capacitor can have a double capacity in principle. Further, while the lithium ion capacitor has a small capacitance compared to the lithium ion battery, the lithium ion capacitor advantageously has a low internal resistance, excellent output characteristics, and a long life [Japanese Unexamined Patent Application Publication No. 2010-141217 (Patent Document 1)]. Patent Document 1 discloses a structure in which a plurality of tabs are used to connect a current collector and a current collecting member to each other.
In order to reliably weld a negative current collector made of a copper foil and a negative current collecting member made of copper during manufacture of such a lithium ion capacitor, laser welding may be performed using YAG laser light [Japanese Unexamined Patent Application Publication No. 2010-93178 (Patent Document 2)]. Patent Document 2 discloses a structure in which a negative current collector is directly welded to a negative current collecting member without using tabs.